The present invention relates to an electromagnetic flowmeter adopting a square wave exciting method for measuring the flow rate of a fluid.
In conventional electromagnetic flowmeters by the square wave exciting method, the square wave current having a low frequency which is an even fraction or even submultiple of the commercial AC power frequency is allowed to flow through an exciting coil of a flow rate detector and the induced voltage to be caused across a pair of electrodes disposed in the fluid flow is sampled, thereby obtaining a so-called flow rate signal including no 90.degree. noise and no in-phase noise. In the electromagnetic flowmeter of this kind, no zero point fluctuation occurs and its operation is stable.
FIG. 1 shows a conventional electromagnetic flowmeter adopting the square wave exciting method. This electromagnetic flowmeter includes a flow rate detector 10 to detect the flow rate of a conductive fluid, and a signal processor 20 to generate a flow rate signal in proportion to the flow rate by processing an output signal of this flow rate detector 10. This flow rate detector 10 includes a tube 11 for allowing the conductive fluid to pass, a pair of electrodes 12 and 13 which are so disposed as to face each other in this tube 11, and a pair of exciting coils 14 and 15 for generating a magnetic flux crossing the tube 11. On the other hand, the signal processor 20 includes a signal source 21 having the commercial power frequency, an excitation control circuit 22 for generating a square wave sampling signal by shaping the waveform of the signal from this signal source 21 and for frequency-dividing this sampling signal, thereby generating an excitation control signal having a frequency equal to an even fraction of the commercial power frequency, an exciting circuit 23 to alternately supply two constant currents having the different polarities or magnitudes from each other to the exciting coils 14 and 15 in response to the excitation control signal from this excitation control circuit 22, an amplifier/inverter circuit 24 for amplifying the voltage to be induced between the electrodes 12 and 13 and inverting the amplified voltage at every half cycle, a sampling circuit 25 to sample an output signal from this circuit 24 in response to a sampling signal from the control circuit 22, and a converting circuit 26 for converting the signal sampled by the sampling circuit 25 into the DC current in a ragne of 4 to 20 mA, thereby generating a flow rate signal proportional to the flow rate.
FIG. 2 shows a practical circuit diagram of the sampling circuit 25. This sampling circuit 25 has an integrating circuit 25A to integrate the signal to be supplied from the amplifier circuit 24 through a switch 25B and a resistor 25C, and a flip-flop 25D (set input high priority) for receiving at its reset input terminal an output signal of the integrating circuit 25A through a comparator 25E. A Q output terminal of the flip-flop 25D is coupled to one input terminal of an AND gate 25F which receives at the other input terminal the sampling signal from the excitation control circuit 22 through an inverter 25G. In addition, the sampling signal from the excitation control circuit 22 is supplied to a set input terminal of the flip-flop 25D. A switch 25H and a reference voltage source 25I are coupled between the ground and the junction between the switch 25B and the resistor 25C. The switches 25B and 25H are controlled by the sampling signal from the excitation control circuit 22 and an output signal from the AND gate 25F, respectively.
In the electromagnetic flowmeter shown in FIGS. 1 and 2, the signal of the commercial power frequency shown in FIG. 3A is supplied from the signal source 21 to the excitation control circuit 22. This control circuit 22 shapes the waveform of this input signal and generates the square wave sampling signal having a frequency equal to the commercial power frequency shown in FIG. 3B, and at the same time, it frequency-divides this sampling signal and generates the excitation control signal shown in FIG. 3C having the frequency equal to an even fraction, e.g., 1/2 of the commercial power frequency. The exciting circuit 23 selectively supplies two constant currents having different polarities or magnitudes to the exciting coils 14 and 15 in accordance with the low or high level of this excitation control signal. Due to this, the alternating magnetic flux is generated between the exciting coils 14 and 15, thereby causing the induced voltage to be generated between the electrodes 12 and 13. This induced voltage is amplified by the amplifier/inverter circuit 24 and is inverted at every half cycle and is supplied to the sampling circuit 25 as the signal voltage shown in FIG. 3D. The sampling circuit 25 samples the output voltage from the amplifier/inverter circuit 24 in response to the leading edge of the sampling signal shown in FIG. 3B, i.e., at a timing when the alternating magnetic field is stable, thereby generating an output voltage in proportion to the flow rate. The voltage sampled in this way is smoothed by the converting circuit 26 and thereafter it is converted into the DC current in a range of 4 to 20 mA.
When the sampling signal at a high level shown in FIG. 3B is supplied from the excitation control circuit 22 to the sampling circuit 25, the switch 25B is closed for the interval corresponding to one cycle of the commercial power frequency, so that the output voltage shown in FIG. 3D from the amplifier/inverter circuit 24 is supplied to the integrating circuit 25A. Due to this, this integrating circuit 25A integrates the output voltage from the circuit 24 for the interval corresponding to one cycle of the commercial power frequency, thereby generating the output voltage which monotonously increases as shown in FIG. 3E. This output voltage of the integrating circuit 25A is supplied through the comparator 25E to the reset input terminal of the flip-flop 25D. Thereafter, when the sampling signal becomes a low level, the flip-flop 25D is set and a high level signal is generated from the AND gate 25F, thereby closing the switch 25H. Due to this, the voltage of the opposite polarity to that of the output voltage from the circuit 24 is supplied from the reference voltage source 25I to the integrating circuit 25A. While the integrating circuit 25A is integrating the output voltage from this reference voltage source 25I, the output voltage of the integrating circuit 25A gradually decreases as shown in FIG. 3E. When it is detected that the output voltage of the integrating circuit 25A has reached the 0 level, the comparator 25E generates an output signal to reset the flip-flop 25D. This allows a low level signal to be generated from the AND gate 25F, thereby turning off the switch 25H. As is well known, the time when the switch 25H is closed is proportional to the level of the output voltage of the circuit 24, while it is inversely proportional to the output voltage level of the reference voltage source 25I. Since the output voltage level of the reference voltage source 25I is constant, it is possible to derive an output signal of the AND gate 25F representing the interval during which the switch 25H is closed as the flow rate signal having a pulse width proportional to the flow rate.
In the above-mentioned electromagnetic flowmeter, since the frequency of the excitation control signal is different from the commercial power frequency, the influence of the noise due to the commercial power source can be minimized. In addition, since the integration sampling interval is set to be equal to one cycle of the commercial power frequency, even if the noise due to the commercial power source is mixed to the flow rate signal as shown in, for example, FIG. 3F, the noise component is eliminated by integrating this signal, so that the influence to the flow rate signal due to the mixture of this noise is minimized.
On the other hand, in the case where the signal source 21 having the commercial power frequency is used to obtain the excitation control signal, the flow rate cannot be measured in case of the failure of power supply. In addition, in case of the occurrence of the instantaneous stoppage of power supply, the exciting cycle and the sampling timing fluctuate, so that this causes a measurement error to be produced. Moreover, the fluctuation of the power frequency influences frequency characteristic of the amplifier to cause a measurment error to be produced.
Therefore, conventionally, a DC driving method is adopted and an oscillating circuit for generating an oscillation signal having the oscillating frequency which is equal to the commercial power frequency and an auxiliary power supply are used.
When other electric power equipment, cables and the like are disposed near the electromagnetic flowmeter, in many cases, the induced noise due to the commercial power source is mixed into an output signal from the flow rate detector 10. In this case, if this commercial power frequency is exactly equal to the oscillating frequency of the oscillating circuit, no particular problem will be caused. However, the commercial power frequency fluctuates in a range of, for example, 48 to 52 Hz in dependance upon the districts or countries. Consequently, it is actually impossible to set the oscillating frequency of the oscillating circuit to be always equal to the commercial power frequency and it is also difficult to make the sampling interval correspond to one cycle of the commercial power frequency. Therefore, in case of using the oscillating circuit, it is difficult to efficiently eliminate the induced noise due to the commercial power source, so that it is difficult to accurately measure the flow rate.